From: Dan Williams <firstname.lastname@example.org> To: Jan Kara <email@example.com> Cc: Christoph Hellwig <firstname.lastname@example.org>, Johannes Thumshirn <email@example.com>, Dave Jiang <firstname.lastname@example.org>, linux-nvdimm <email@example.com>, Linux MM <firstname.lastname@example.org>, linux-fsdevel <email@example.com>, linux-ext4 <firstname.lastname@example.org>, linux-xfs <email@example.com>, Linux API <firstname.lastname@example.org> Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps Date: Wed, 3 Oct 2018 08:13:37 -0700 Message-ID: <CAPcyv4iJvN6_Cf6tw=5a=Uh99LfMFKU7n8QkGcz1ZaxL0Oiemail@example.com> (raw) In-Reply-To: <20181003150658.GC24030@quack2.suse.cz> On Wed, Oct 3, 2018 at 8:07 AM Jan Kara <firstname.lastname@example.org> wrote: > > On Wed 03-10-18 07:38:50, Dan Williams wrote: > > On Wed, Oct 3, 2018 at 5:51 AM Jan Kara <email@example.com> wrote: > > > > > > On Tue 02-10-18 13:18:54, Dan Williams wrote: > > > > On Tue, Oct 2, 2018 at 8:32 AM Jan Kara <firstname.lastname@example.org> wrote: > > > > > > > > > > On Tue 02-10-18 07:52:06, Christoph Hellwig wrote: > > > > > > On Tue, Oct 02, 2018 at 04:44:13PM +0200, Johannes Thumshirn wrote: > > > > > > > On Tue, Oct 02, 2018 at 07:37:13AM -0700, Christoph Hellwig wrote: > > > > > > > > No, it should not. DAX is an implementation detail thay may change > > > > > > > > or go away at any time. > > > > > > > > > > > > > > Well we had an issue with an application checking for dax, this is how > > > > > > > we landed here in the first place. > > > > > > > > > > > > So what exacty is that "DAX" they are querying about (and no, I'm not > > > > > > joking, nor being philosophical). > > > > > > > > > > I believe the application we are speaking about is mostly concerned about > > > > > the memory overhead of the page cache. Think of a machine that has ~ 1TB of > > > > > DRAM, the database running on it is about that size as well and they want > > > > > database state stored somewhere persistently - which they may want to do by > > > > > modifying mmaped database files if they do small updates... So they really > > > > > want to be able to use close to all DRAM for the DB and not leave slack > > > > > space for the kernel page cache to cache 1TB of database files. > > > > > > > > VM_MIXEDMAP was never a reliable indication of DAX because it could be > > > > set for random other device-drivers that use vm_insert_mixed(). The > > > > MAP_SYNC flag positively indicates that page cache is disabled for a > > > > given mapping, although whether that property is due to "dax" or some > > > > other kernel mechanics is purely an internal detail. > > > > > > > > I'm not opposed to faking out VM_MIXEDMAP if this broken check has > > > > made it into production, but again, it's unreliable. > > > > > > So luckily this particular application wasn't widely deployed yet so we > > > will likely get away with the vendor asking customers to update to a > > > version not looking into smaps and parsing /proc/mounts instead. > > > > > > But I don't find parsing /proc/mounts that beautiful either and I'd prefer > > > if we had a better interface for applications to query whether they can > > > avoid page cache for mmaps or not. > > > > Yeah, the mount flag is not a good indicator either. I think we need > > to follow through on the per-inode property of DAX. Darrick and I > > discussed just allowing the property to be inherited from the parent > > directory at file creation time. That avoids the dynamic set-up / > > teardown races that seem intractable at this point. > > > > What's wrong with MAP_SYNC as a page-cache detector in the meantime? > > So IMHO checking for MAP_SYNC is about as reliable as checking for 'dax' > mount option. It works now but nobody promises it will reliably detect DAX in > future - e.g. there's nothing that prevents MAP_SYNC to work for mappings > using pagecache if we find a sensible usecase for that. Fair enough. > WRT per-inode DAX property, AFAIU that inode flag is just going to be > advisory thing - i.e., use DAX if possible. If you mount a filesystem with > these inode flags set in a configuration which does not allow DAX to be > used, you will still be able to access such inodes but the access will use > page cache instead. And querying these flags should better show real > on-disk status and not just whether DAX is used as that would result in an > even bigger mess. So this feature seems to be somewhat orthogonal to the > API I'm looking for. True, I imagine once we have that flag we will be able to distinguish the "saved" property and the "effective / live" property of DAX... Also it's really not DAX that applications care about as much as "is there page-cache indirection / overhead for this mapping?". That seems to be a narrower guarantee that we can make than what "DAX" might imply.
next prev parent reply index Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-02 10:05 Jan Kara 2018-10-02 10:50 ` Michal Hocko 2018-10-02 13:32 ` Jan Kara 2018-10-02 12:10 ` Johannes Thumshirn 2018-10-02 14:20 ` Johannes Thumshirn 2018-10-02 14:45 ` Christoph Hellwig 2018-10-02 15:01 ` Johannes Thumshirn 2018-10-02 15:06 ` Christoph Hellwig 2018-10-04 10:09 ` Johannes Thumshirn 2018-10-05 6:25 ` Christoph Hellwig 2018-10-05 6:35 ` Johannes Thumshirn 2018-10-06 1:17 ` Dan Williams 2018-10-14 15:47 ` Dan Williams 2018-10-17 20:01 ` Dan Williams 2018-10-18 17:43 ` Jan Kara 2018-10-18 19:10 ` Dan Williams 2018-10-19 3:01 ` Dave Chinner 2018-10-02 14:29 ` Jan Kara 2018-10-02 14:37 ` Christoph Hellwig 2018-10-02 14:44 ` Johannes Thumshirn 2018-10-02 14:52 ` Christoph Hellwig 2018-10-02 15:31 ` Jan Kara 2018-10-02 20:18 ` Dan Williams 2018-10-03 12:50 ` Jan Kara 2018-10-03 14:38 ` Dan Williams 2018-10-03 15:06 ` Jan Kara 2018-10-03 15:13 ` Dan Williams [this message] 2018-10-03 16:44 ` Jan Kara 2018-10-03 21:13 ` Dan Williams 2018-10-04 10:04 ` Johannes Thumshirn 2018-10-02 15:07 ` Jan Kara 2018-10-17 20:23 ` Jeff Moyer 2018-10-18 0:25 ` Dave Chinner 2018-10-18 14:55 ` Jan Kara 2018-10-19 0:43 ` Dave Chinner 2018-10-30 6:30 ` Dan Williams 2018-10-30 22:49 ` Dave Chinner 2018-10-30 22:59 ` Dan Williams 2018-10-31 5:59 ` y-goto 2018-11-01 23:00 ` Dave Chinner 2018-11-02 1:43 ` y-goto 2018-10-18 21:05 ` Jeff Moyer 2018-10-09 19:43 ` Jeff Moyer 2018-10-16 8:25 ` Jan Kara 2018-10-16 12:35 ` Jeff Moyer
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